Recent preprints on high anharmonicity flux circuits

We posted three preprints describing recent work on quantum control of high anharmonicity flux quantum circuits. This work is based on a new design of a capacitively shunted flux circuit ( ) that combines large level anarhmonicity and long energy relaxation times. We characterized the coherence in the qubit subspace; we find energy relaxation times of about 40 us and dephasing times with dynamical decoupling in the tens of microsecond range. We also characterized decoherence in the subspace of the lowest three states of the system.

We implemented a qutrit fundamental gate – the Walsh Hadamard gate using a new approach based on a single pulse decomposition enabled by simultanous driving of all transitions in the qutrit ( ), with a high fidelity of 99.2 %.

More recently, we demonstrated randomized benchmarking characterization of control of the qutrit ( Randomized benchmarking is routinely used in qubit gate characterization. This work is a demonstration of benchmarking for a qutrit. It is enabled by the application of a universal gate decomposition method for qudits. The work sheds new light on control issues that are unique to multileve systems and opens new avenues for control of qutrits.

Quantum measurements in a different basis

A paper with collaborators from Saarland University has just been published in Physical Review A ( ). In this paper we propose a new method to perform quantum measurements of superconducting qubits. Common measurements of superconducting qubits work in the energy basis. At a fundamental level, this is due to the fact that the dynamics during measurements is dominated by the qubit dynamics. In our paper we propose a new protocol, which allows measurements in a different basis, based on an indirect measurement scheme. This work has relevance for quantum annealing and other areas of quantum information.

Paper on cutoffs and atom shapes for superconducting circuits published in Physical Review A

Our paper Finite sizes and smooth cutoffs in superconducting circuits was recently published in Physical Review A. In this paper we discuss the strong interaction between an atom and a one dimensional electromagnetic field, a regime that we have demonstrated in recent experiments. We discuss the role of atomic shape and of cutoff function for coupling to fields. We find that the cutoff scale type is highly relevant in superconducting circuits and observable in experiments measuring qubit spontaneous excitation or decay by the field vacuum.

New preprint on quadrature measurement and squeezed states

Quadrature operators for a harmonic oscillator have the property that they can be measured in a quantum non-demolition way, that is with a precision only limited by the measurement apparatus. This property makes quadrature measurements relevant in quantum sensing, where small signals acting on the harmonic oscillator can be detected by monitoring a quadrature. A topic closely related to quadrature measurements is that of squeezed states, which have reduced uncertainty in one quadrature. Squeezed states are important for enabling quantum sensing and for quantum information and optics in general.

In our recent preprint, we discuss a method to implement quadrature measurements and generate squeezed states. This method is versatile due to the fact that it relies on using a qubit as a detector, an elementary resource available in many physical implementations. This method has promising prospects for experimental implementation with either nanomechamical or electrical resonators.

Paper on ultrastrong atom-field interactions in Nature Physics

Our paper, Ultrastrong coupling of a single artificial atom to an electromagnetic continuum in the nonperturbative regime, was accepted in Nature Physics and has appeared as an advanced online publication on October 10, 2016. A News and Views article by Kater Murch discusses our result and two related papers on superconducting artificial atoms in interaction with a cavity and a photonic crystal respectively. See also this news article discussing this result. Congratulations to Pol and the others for this work!